The presently disclosed subject matter relates to a TWA measuring apparatus and TWA measuring method which can accurately measure the presence of TWA (T-wave alternans).
TWA appears at onset of illness such as QT prolongation syndrome, variant angina, acute myocardial ischemia, electrolyte abnormality, paroxysmal tachycardia, bradycardia, or pericardial fluid accumulation. TWA is a phenomenon in which the amplitude and polarity of the T wave appearing in an electrocardiogram are alternately changed, and an index effective to predict sudden cardiac death. TWA is not a phenomenon which can be always observed with the naked eye, and therefore its application in clinics is limited.
From the 1980s, consequently, techniques for enabling minute TWA (Microvolt TWA: MTWA) to be measured by a computer have been developed.
Examples of currently proposed techniques for measuring TWA are a measurement technique based on the MMA (Modified Moving Average) method of General Electric (GE) Company, and that based on the periodogram of Cambridge Heart (CH), Inc. which are disclosed in U.S. Pat. No. 6,668,189 and U.S. Pat. No. 5,935,082, respectively.
The measurement technique of GE Company is directed to a method of analyzing a time waveform in a time region, and is said to have resistance to noises. However, the technique does not have a long history as a measurement technique, and it is required to watch its clinical effect.
By contrast, the measurement technique of CH Inc. which is a technique in a frequency region has been used from the 1980s, and hence its effectiveness in clinics has been proved. Today, therefore, it is considered that the measurement technique based on the periodogram of CH Inc. is clinically more useful than that based on the MMA method of GE Company.
With respect to the measurement technique based on the periodogram of CH Inc., after its announcement, various techniques for performing new processes, such as a technique of measurement electrodes are added, and still now the added latest techniques have been used.
In the technique of measuring TWA, a value (alternans voltage) indicative of the magnitude of TWA, and a value (alternans rate) indicative of the reliability of the measurement of TWA are calculated, and, when the magnitude of the value indicative of the reliability of the measurement of TWA is equal to or larger than 3.0, it is determined that TWA exists.
The index in which the magnitude of the value indicative of the reliability of the measurement of TWA is equal to or larger than 3.0 was derived based on experimental results. In the case where noises are mixed in measured electrocardiogram waveforms, even when TWA exists, therefore, it is often that the magnitude of the value indicative of the reliability of the measurement of TWA is not equal to or larger than 3.0, and the determination is determined to be impossible.
As a result of a test for evaluating the existing technique of measuring TWA, it has been known that the rate of the determination which is determined to be impossible is from 10% to 40% (see is Microvolt T-wave Alternans the Answer to Risk Stratification in Heart Failure?: Circulation: 2007: 116: 2984-2991). With respect to a positive herd in which a group that is determined to be indeterminable is added to a group that is determined that TWA exists, and a negative herd configured by only a group that is determined that TWA does not exist, their prognoses (medical expectations for the disease process) were observed, and it has been found that the prognosis of the positive herd is worse than that of the negative herd (see Prognostic Value of T-wave Alternans in Patients With Heart Failure Due to Nonischemic Cardiomyopathy: J. Am. Coll. Cardiol: 2007: 50: 1896-1904).
Therefore, it is anticipated that a patient having a poor prognosis exists among patients who are determined to be indeterminable, and confusions occur in the interpretation of a determination result of indetermination.